Prevention and intensification of chemically assisted meltwater interactions

Abstract

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.

A vapor explosion (VE) is a thermo-fluid interaction phenomenon in which a hot liquid (e.g. molten metal) transfers its thermal (and possibly also chemical) energy to a cold vaporizing liquid (e.g. water) over an explosive time scale. VE’s are practically relevant to a variety of industrial processes, including the metals casting, pulp-paper, volcanology, liquid natural gas, and nuclear industries. This paper investigates the potential for suppressing, and importantly, for intentionally enhancing the energetics of spontaneous and on-demand triggered VEs. Energetic enhancements are possible by coupling the exothermic oxidation reaction between aluminium and water with the explosive fragmentation produced by a VE. Experiments were conducted with two model types of hot melts: (a) samples of Sn, which freezes at 232 °C, to serve as a baseline comparison for purely thermal heat driven explosions, and (b) various alloys of Al-GaInSn, which remain liquid even at 20 °C. Al-GaInSn alloys are additionally known for their ability to evolve hydrogen from water at room temperature, but over a long (non-explosive) timescale. Thus, Al-GaInSn alloys may serve as a model for combined thermal (sensible heat) and chemical energy explosions. Spontaneous explosions were successfully achieved in the Al-GaInSn-H2O system through the systematic, passive manipulation of the water chemistry and temperature. Spontaneous explosions, on the other hand, could be convincingly suppressed (100% of the time) through the introduction of non-condensable gases within the hot-cold fluid interfacial vapor layer. Means for chemically assisted and induced explosions were also devised. Specifically, for transforming a previously inert system to an explosive state, the combination of 5 w/o aqueous NaCl and 5 °C watertemperature provided such an outcome. Distinctly more violent explosions could be reliably triggered using an underwater shock-producing detonator to forcibly destabilize melt-water systems of varied compositions. High speed photography reveals significant enhancements (as compared to the case where explosions are not actively triggered) in the rate of hydrogen production milliseconds after onset of the explosive event.